The present invention relates to a flowmeter for measuring the velocity of flow of a fluid, such as, a liquid, gas or vapour phase, by detecting the vortex shedding frequency of the fluid flow and, more particularly, to such a flowmeter which utilises a fibre optic sensing element to detect vortex shedding.
Flow rate measurement based on the natural phenomenon of vortex shedding is known and has been realised using a variety of techniques for monitoring the vortex shedding frequency. When a bluff (that is, non-streamlined) body is sited in a fluid flow, it will, under certain conditions, cause a regular stream of vortices to form in the fluid downstream of the body. These vortices leave alternately from opposite sides of the body. Moreover, as each vortex is generated and shed, it produces a lateral force on the body and, if the body is sufficiently flexible, it will oscillate.
The pattern of flow past the bluff body is governed by the Reynolds number (Re). The transition from a steady laminar flow to an unsteady turbulent flow occurs at Re.about.40. For Re&gt;40 wakes appear behind the body and eddies are formed. When Re exceeds 100, the boundary layer separates and the eddies are shed alternately from opposite sides of the body, resulting in the Karman vortex sheet. This vortex shedding occurs with a regular periodicity in the turbulent flow region, except in the transition regions when 200&lt;Re&lt;400 and 3.times.10.sup.5 &lt;Re&lt;3.times.10.sup.6. This breakdown is mainly due to transition from a laminar to a turbulent boundary layer state.
The vortex shedding frequency f is related to the flow velocity v by the equation EQU f=sv/d
where s is the Strouhal number and d is the diameter of a cylindrical bluff body. Generally s is a function of the Reynolds number, and the study of the flow velocity dependence of s is an established branch of research in hydrodynamic studies. For large values of Re the turbulence of the vortices has a stabilising effect on the value of s and it is effectively constant. Hence, the flow velocity may be directly determined from a measurement of the vortex shedding frequency.
The linearity and wide dynamic range of the vortex shedding process have been exploited in a number of commercial flowmeter designs. Vortex shedding detection techniques include temperature, pressure and strain sensing, with the sensing element either being sited on the bluff body or being disposed down the vortex sheet. Also, a flowmeter of this type has been proposed which uses a multimode optical fibre sensing element, as the bluff body, and in which the oscillating strain induced in the fibre sensing element, as a result of the vortex shedding effect, is detected by the fibredyne technique. This proposal is described in the publication "Electronics Letters" of Mar. 19, 1981 at page 244. The fibredyne technique suffers from random fading and the generation of large numbers of harmonics of the fundamental pertubation. Whilst the technique adequately determines the vortex shedding frequency, it shows the harmonics of the fundamental frequency in the output spectrum and does not give the absolute amplitude of the strain.